Closes pydata#8090
Closes pydata#8206
Closes pydata#9398

* main: (26 commits)
  Forbid modifying names of DataTree objects with parents (pydata#9494)
  DAS-2155 - Merge datatree documentation into main docs. (pydata#9033)
  Make illegal path-like variable names when constructing a DataTree from a Dataset (pydata#9378)
  Ensure TreeNode doesn't copy in-place (pydata#9482)
  `open_groups` for zarr backends (pydata#9469)
  Update pyproject.toml (pydata#9484)
  New whatsnew section (pydata#9483)
  Release notes for v2024.09.0 (pydata#9480)
  Fix `DataTree.coords.__setitem__` by adding `DataTreeCoordinates` class (pydata#9451)
  Rename DataTree's "ds" and "data" to "dataset" (pydata#9476)
  Update DataTree repr to indicate inheritance (pydata#9470)
  Bump pypa/gh-action-pypi-publish in the actions group (pydata#9460)
  Repo checker (pydata#9450)
  Add days_in_year and decimal_year to dt accessor (pydata#9105)
  remove parent argument from DataTree.__init__ (pydata#9465)
  Fix inheritance in DataTree.copy() (pydata#9457)
  Implement `DataTree.__delitem__` (pydata#9453)
  Add ASV for datatree.from_dict (pydata#9459)
  Make the first argument in DataTree.from_dict positional only (pydata#9446)
  Fix typos across the code, doc and comments (pydata#9443)
  ...

* main:
  Opt out of floor division for float dtype time encoding (pydata#9497)
  fixed formatting for whats-new (pydata#9493)